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a simple explaination of agate formation

Posted by Daniel Bennett  
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Daniel Bennett February 13, 2017 03:30PM
picture if you will oil and water shook up with little balls of oil swirling in water. but instead liquid quartz balls swirling in liquid magma. like a red hot stone dropped in cold water liquid magma trembles and shakes as it surfaces to atmospheric temp. the bands in agates form when vibrations from rapidly cooling basalt cause "ripples" in the solidifying quartz balls. like moving water vibrating quartz takes longer to" freeze" so it freezes up layer by layer starting from the outside. in the case of so called "water level agates" when the magma first surfaces vibrations are coming from all direction and a little later the majority of vibrations start coming from below. by that time the outermost part of the agate is solidifying but the center is still fluid and so the perimeter bands turn in to flat parallel bands. which also points towards water level agates coming from the uppermost parts of the flow. agate "eyes" spherolites are from a higher temp melting point particle present in the quartz during the vibrations often finding its way to the outside edge while any gas bubble present will tend to gravitate to the center...
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Doug Daniels February 14, 2017 02:06AM
Interesting idea, except that quartz does not form from a melt that forms basalt - only after later weathering does the quartz get deposited. And, doesn't explain agates that form in rhyolitic rocks. Or in sediments.
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Ralph Bottrill February 14, 2017 04:52PM
Yes, quartz is not immiscible in most magmas, so will usually react and intermix quite rapidly. It's quite well established that the agate is formed post-cooling by precipitation from low temperature hydrothermal solutions carrying dissolved silica.
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Gregg Little February 15, 2017 12:15AM
I wasn't aware that vibrations and rapid cooling basalt was the environment of agate formation. My understanding was that it was largely post solidification and in the much lower temperature regime. To bring in the chalcedony forming fluids, fracturing by shrinkage and circulating waters were needed. Am I missing something recent with the vibration concept?
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Daniel Bennett February 15, 2017 12:27AM
i have to confess i don't know any of that to be true. it just seems reasonable and logical. apparently not so. why do they find little quartz/chalcedony nodules in Hawaii where the basalt is so fresh. doesn't it take a long time to happen the way you guys are talking about?
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Donald Kasper February 28, 2017 06:56AM
Agates form in volcanic rocks or volcanic ash related to volcanism, only. So-called sedimentary agates are found under ash beds that was the silica source. Agates form in supercritical fluids. As magma comes to the surface, the feldspar and silica separate from each other. The silica under certain conditions forms agate in conjunction with supercritical fluid. The lower the silica content of the magma, the less agates are found. Silica and rhyolite intermix. Andesites have vein agates. Basalts have amygdules. It is not the net content of silica that is the sole driver, it is the catalyst to force it out of solution that also matters. Agates are calcite-clay-silica-hydrate rocks. They have many species of opal, moganite, quartz, cristobalite. They are commonly included with calcite which plays a key role in their formation by creating alkaline conditions. The inclusions and shell structures are commonly clays. Celadonite and bentonite are the most common. Infrared spectroscopy equipment I have shows for example that the Union Road Missouri agates all have celadonite shells. Even though the host is sedimentary, celadonite cannot form from weathering. It forms only in volcanic rocks and their ash equivalents. Snakeskin agates have shells of bentonite and celadonite glaze, which disproves their magadiite precursor and weathering model. With infrared, no zeolites, no salts, no evaporites are found in agates. Sulfides are rare and only occur with a few types that can form in neutral pH systems. They form in agates when calcite is present. The calcite forms sulfate and neutralizes the acidity. Overall, the inclusions tell us their temperature of formation. Chlorite and celadonite found in them are formed around 425 C and not with weathering. The glass state of rhyolite between hard rock and melt is the range of 374 to 575 C. Above 374 C is only supercritical water. A fluid gas with the solubility of water and diffusion of a gas. No surface tension exists. In this system, the charge on silica is negative, and you will note that only cation positively charged minerals or metals are found in agates. Anion mineral states are never found. Groundwater does not dissolve silica and it does not just move around to and fro. It takes alkaline systems to dissolve silica. So, agates are rock, they are not varietal quartz, and they form very quickly (hundreds of years max time scale), and only in volcanic rocks. In fact, they are almost never found near water, near rivers, water tables, or lakes, or river mouths. They are typically found in deserts. You never go to the tropical rainforests of Brazil to hunt agates, you go to the Mojave Desert of California or Sahara of Morocco. This defeats a weathering water model for rocks not found with water. Where does their water come from? Right out of the melt. It is volcanic water, and not hydrothermal water. Hydrothermal water is rich in calcite, not silica. To get pure silica, you cannot have groundwater. Agates have no humic acid to show they were in contact with ground water. Humic acid is not found in agates cores in infrared, just the shells. That comes from weathering, not agate formation. As you look closely, no popular model of weathering has any scientific basis to be believable as no data fits a weathering model.



Edited 2 time(s). Last edit at 02/28/2017 07:01AM by Donald Kasper.
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Donald Kasper February 28, 2017 07:03AM
Most models of geologic formation of agates pick and chose what fits their concept of formation. All of the data must be taken into account. Inferior models are easily disproven. For example, granites have 30% quartz, have voids, and are exposed to weathering. Show me one agate in one granite of this planet. You cannot. Therefore the weathering model just dropped dead. Having quartz around to erode does not work. Quartz does not weather. It must be mechanically broken down. Germany has Permian age geodes with kaolinized lava and pure agate cores that are totally unaffected. If exposed to weathering in very wet conditions, the Permian agates are 100% intact and filled with kaolinite in the fiber bands as the bands are porous. Agates are not quartz layers. They are groups of fiber bands of quartz and moganite, and occasional layers of pure crystal quartz. There are also microspheres in the agates. Inferior technology based on refractive index in the 1800's presumed these were opals. Infrared spectroscopy and Raman spectroscopy proves these are moganite structures. So you have one moganite in the fibers, and another immiscible form. Let me see. The moganite in the fibers makes a 555 cm-1 band, and the other moganite microspheres does not. Raman reports both are moganite. Perhaps one is alpha-moganite and the other is beta-moganite. We have now found our beta-moganite, only found in ignimbrite rocks or rocks exposed to pyroclastic flows. In essence, a galaxy of complexity reduced to stupidity for bland generalizations. The tubes making moss structures are not bacteria unless you can show a bacteria that precipitates celadonite, because that is what infrared shows them to be.



Edited 2 time(s). Last edit at 02/28/2017 07:10AM by Donald Kasper.
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Jolyon & Katya Ralph February 28, 2017 09:51AM
> So-called sedimentary agates are found under ash beds that was the silica source.

Not true. There are plenty of examples of agate of entirely sedimentary origin, where biogenic silica (eg from radiolaria/diatoms) is the primary source.
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Ralph Bottrill February 28, 2017 10:16AM
Agates only found in deserts? I can think of dozens of locations, without really trying, in non desert climates, including Scotland, Germany, Tasmania, Queensland, Brazil, etc.
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Larry Maltby February 28, 2017 03:16PM
“Agates form in volcanic rocks or volcanic ash related to volcanism, only.”

This is an extremely absolute statement. It leaves no “wiggle room”. It implies that all of the complex geological processes in the world have been investigated and this is the conclusion. The formation of agate remains as a highly controversial subject with some good points on all sides of the debate but as yet it is far from a conclusion. It seems to me that agates sometimes form in sedimentary deposits as a chemical process that is separate and apart from volcanism. It may well be that the silica in a sedimentary agate was derived from the decomposition of an overlaying layer of volcanic ash but the ash fall itself is a sedimentary process and the decomposition of the ash that frees the silica is not a volcanic process. There are other sources in sedimentary deposits that may provide silica to support the formation of agate such as plankton, diatoms etc.

Really, there is no such thing as a simple explanation of agate formation!



Edited 2 time(s). Last edit at 02/28/2017 03:37PM by Larry Maltby.
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Gregg Little February 28, 2017 05:12PM
I find some strange and down right erroneous statements, oh dear where do I start;

1). "Agates form in volcanic rocks or volcanic ash related to volcanism, only." The adjective "volcanic" can only mean from volcanism and its processes.

2). "Silica and rhyolite intermix. Andesites have vein agates. Basalts have amygdules." The void spaces that agate forms in is a function of tectonic stresses that create the void space, not a function of agate formation.

3). "Agates are calcite-clay-silica-hydrate rocks." If you look in any book on mineralogy they all basically say "agate is a banded form of finely-grained, microcrystalline quartz". All other constituents are impurities, inclusions, etc. Agate is quartz.

4). "Groundwater does not dissolve silica and it does not just move around to and fro. It takes alkaline systems to dissolve silica." Since when does groundwater that becomes alkaline cease to be ground water? Sedimentary basins around the world can have and have alkaline waters. In my petroleum geology work have seen chalcedony (aka agate) in sedimentary rocks. I have also seen stylolites (pressure solution features) in quartz arenites (sandstones) which puts silica in solution which then precipitates as quartz overgrowths on quartz grains.

5). "So, agates are rock, they are not varietal quartz" Is Donald Kasper trying to dismiss the whole field of geological science?

6). "They are typically found in deserts.". As Ralph B. stated and I reiterate British Columbia and Nova Scotia are not deserts, but a source of agate in volcanic terrains in a temperate climatic zone.

7). "no popular model of weathering has any scientific basis ". I am not sure how weathering was brought up except by Donald Kasper; weathering is post depositional.

8). "Most models of geologic formation of agates pick and chose what fits their concept of formation.". See my comment #5. Geologists in research do not pick and choose, they use the volumes of past research work and their evidence to support their research thesis.

9). "Quartz does not weather. It must be mechanically broken down.". Weathering is both a mechanical and chemical process, again check the literature. Many, many water analyse show dissolved silica.

And last but probably not finally, 10). "In essence, a galaxy of complexity reduced to stupidity for bland generalizations." Agate formation definitely has "a galaxy of complexity" but the latter part of this statement appears to be what you are advocating.



Edited 1 time(s). Last edit at 02/28/2017 05:14PM by Gregg Little.
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Alfred L. Ostrander March 01, 2017 02:51PM
Greg, very precise points. You beat me to several points but I will comment on the general quartz family of which agate has long been accepted as a variety. No need to upset the apple cart there as the different varieties have long been established. We will learn more but a radical overturn of the quartz group is highly unlikely. Simply saying that the presence of various phases in a sample along with some impurities makes it a rock is really an over simplification. By that reasoning any specimen with several minerals forming together and some impurities can be called a rock. So all those beautiful well crystallized specimens in our collections are reduced to being just rocks? There is truth to that statement but not in context.

As to not collecting agates in the tropical rain forests of Brazil, think of the agates of Rio Grande Do Sul, Brazil. Although it is in the southern temperate zone it is humid and sub-tropical and forested. People go collect agates in areas with little or no vegetative cover because they are easier to see. That is all. Except for walking some beaches on Lake Superior. Or collecting amidst the pines at Teepee Canyon, South Dakota. Not desert and great agates can be found.

To Daniel, maybe we as geologists need to be more careful when speaking of geologic time. It runs from billions of years to the present. All geologic processes do not run at the same speed throughout all time. How long does it take to grow a quartz crystal in a lab? That does shed light on what many geologists once thought would take a far far longer amount of time. We do keep learning! I am now in my mid sixties.When I was young I thought someone that old was truly ancient. My thinking has changed: time needs to be kept in perspective.
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Michael Hatskel March 01, 2017 04:18PM
Speaking of time...
Donald Kasper stated that agates "form very quickly (hundreds of years max time scale)".
Could someone please comment on whether the growth rate data is available that supports that?

Disclaimer: I am certainly not trying to figure out how to grow baseball-sized agates. :-) Just interested in the process kinetics.
A quick Google search on the subject returned the Min Mag 2009 paper titled "Crystallite growth kinetics in nanocrystalline quartz (agate and chalcedony)" [http://minmag.geoscienceworld.org/content/73/4/551]. It may have some data regarding the growth rates. I would appreciate if someone could send me the PDF.

Thanks in advance



Edited 1 time(s). Last edit at 03/01/2017 04:18PM by Michael Hatskel.
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Gregg Little March 01, 2017 09:58PM
Alfred;
Being of similar "vintage" and speaking of long duration processes, I remember a pre-eminent researcher speaking at my university on a recently accepted theory called continental drift. Long time ago for me, a nano blink of geological time.

Getting back to crystal formation, one of our recent threads was about inclusions and described growing small centimetre-size quartz crystals in a lab over 4 months using seed crystals. I imagine geological processes can match or exceed that given the right conditions. Part of the many variables would include favorable and stable fluid chemistry, high volume, constant temperature and pressure for the duration, etc, etc.

One thing to keep in mind too is that crystals can go through growth phases (ie phantoms) so that further complicates bracketing the time for formation. We humans seem to be time obsessed but the more important question geology asks is, what are the specifics that lead to crystal formation. Time usually is expressed in dating the preceding, concurrent and following processes around the time of crystal formation conditions. These time brackets are usually in the thousands to millions of years and don't necessarily describe when that particular crystal in your hand began and completed its growth.

One of the interesting time frames relates to diamond. It probably takes millenniums to eons for them to form, 10's of miles below the surface, but some of the literature describes their emplacement in kimberlite pipes to be in as little a period as days.
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Rick Dalrymple March 01, 2017 11:04PM
Donald,
I have agate nodules that have formed in shale--directly. They are clearly not formed from volcanic ash or even under an ash.

Why do you think they can only for as a result of vulcanism?

Rick
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Donald Kasper March 02, 2017 03:20AM
It is quite well established that geodes have agates cores and that the plastic state of rhyolite is 374 to 575 C. Above that is a melt and below that is cold rock that only brecciates. So this is the temperature they formed at with silica migrating to the core and the exsolution and expulsion of feldspar. Studies that say agates are formed at 35 C are based on O-18/O-16 ratios which can say anything you like as migration of water with salt changes the ratio and the presumed temperature. In fact, minerals formed at volcanic spreading center trenches are mapped at this temperature by this method while the lava erupts at 1250 C. In terms of chlorite and celadonite, they form directly in volcanic systems and not from weathering and are found in agates consistently. More celadonite than chlorite.

There is a few problems with weathering models. First, all amygdules and geodes have an outer lining of silica in every agate of this planet. Groundwater cannot deposit silica films to do this. Groundwater will make laminar silica structures only, wall-to-wall. This so-called sticky physics to line all geodes/amygdules on earth does not exist with water. There is only one physical state where the voids can be lined uniformly with silica and that is called vapor phase deposition which only occurs in supercritical fluids.

Then you have that small problem with interior waterlines and vapor tops with crystal quartz or banded agate. Blow it off as coincidence as you wish, but if you want an actual model of the physics by which this occurs, weathering does not work. However, when the lava with its silica cools below 374 C and goes subcritical, the system must form two phases. One is water and the other is water vapor. Supercritical fluid and silica going subcritical form waterline floors and vapor phase crystal or banded tops is prima facie proof of a supercritical transition occurrence.

When rocks are exposed to ash only in sedimentary strata, those agates form only in subcritical systems. These are totally different, sinuous and concave structural agates and never have waterlines. These form at or around 150 C. There is no such thing as the one method of agate formation. There are the 70 geologic systems of formation with common themes.

Acicular morphologies of minerals are not accidents of nucleation. Acicular morphologies only form in supercritical fluids where there is no surface tension. There are 4 geologically important supercritical fluid types, each with its own transition temperature. Morphologies (shape-types) are treated in the literature as accidents of God, but are formed in very specific geochemical conditions.

Transition times from super- to subcritical matters and can oscillate depending on heat sources in the volcanics. Decompression in transition causes boiling where the fluid starts to cycle up the walls and drip down the center. This forms the silica pendant structures, always and only perpendicular to the horizontal waterlines.

So we can go through every major agate structural morphology and define it in terms of supercritical transit times and types. For example, rapid subcritical transitioning causes an implosion to occur, and sucks in the exterior volcanic rock around the void structure. When the shell celadonite comes in, its water is expelled. Anywhere water contacts these silica fluids, opal is formed. So the celadonite filaments are encased in opal.

If I am correct in identifying beta-moganite in infrared, this mineral forms at the lower supercritical transition start at 354 C. As such, I have only two populations of moganite, no intermediate forms, with the beta only found in the hottest pyroclastic rocks. Both populations are known to be moganite in Raman infrared, as specimens shared with NASA scientists have found. If correct, and I do collaborate with some planetary scientists at JPL Pasadena on this model, then your agates are filled with beta-moganite and your groundwater model drops dead. it occurs in a variety of very specific structures and is different than moganite in banding layers. These two populations of moganite have totally different spectral behaviors. That is not a conclusion based on 4 rocks like say, oh, Moxon might publish on, but based on 35,000 total agate and volcanic rock graphs with a set of 2500 moganite graphs to study.



Edited 1 time(s). Last edit at 03/02/2017 03:22AM by Donald Kasper.
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Donald Kasper March 02, 2017 03:37AM
This is provably false.

The Union Road Missouri agates of sedimentary source rock scan in infrared with shells of celadonite. Celadonite cannot form from weathering.

That site sits on a fault and was subjected to Western orogency volcanic ash deposition as its silica source.

Subsea volcanic eruptions do two things. First they dump ash into the sea. Second they cause a radiolaria bloom. Third the silica settles to the sea floor and the radiolarian bloom dies out and settles down as well. Then you get say, a Mookaite, found in 7 beds of volcanic ash with radiolaria in a bentonitc chert. So then smart PhD's conclude radiolaria make agate and chert. No. Ash makes the agate and chert and radiolaria.

The Keokuk Geodes are not sedimentary. They are found under a bentonite bed with radiolara in the shells and kaolinite.

The Lake Magadii, Kenya chert, presumed to form by magadiite, is found by drill core under 45 feet of volcanic ash and 100 feet under the magadiite.

Magadii-type cherts from Rome, Oregon has crust shells of beta-moganite, bentonite, and celadonite. If I am correct in identifying beta-moganite in infrared, then they formed at or above 354 C. There are specific infrared reasons to make this presumption of moganite beyond discussion here, but my infrared has been calibrated in two blind studies to NASA Raman infrared from where moganite is defined. It is not defined by XRD, it is defined by what a Raman instrument says at 501 cm.

The sedimentary Dryhead agates are covered in celadonite and bentonite shells with the quartz/chert. You have to study the 4300 cm-1 phyllosilicate region in infrared to see it, or you just won't find it.



Edited 1 time(s). Last edit at 03/02/2017 03:39AM by Donald Kasper.
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Donald Kasper March 02, 2017 03:49AM
You don't know what you have until you have a method of spectroscopy to scan it. Otherwise, you are guessing. Send me a sample, and I will scan it. All sedimentary sites with agate and cherty agate is overlain by bentonite. You just have to look. For older rocks, the looking is harder. It took longer to find the bentonite in the Kentucky agate geology, for example. But, armed with a detailed master key I worked out for 4300 cm-1 infrared clay minerals, I can see the clays despite their trace occurrence in silica rocks. it is the same spectral region used in planetary remote sensing and works for very specific physical reasons.
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Donald Kasper March 02, 2017 03:59AM
We can estimate agate formation by understanding they only form in lava flows and pyroclastic rocks. So we know the cool-down time of those flows based on their type and mass (thickness). So we can make estimates of formation time. So, you have an ash flow 30 meters thick. You have a certain cool down rate. This has been studied for Mono Lake flows. Then you take a theory of formation at 374 to 575 C which is both a rhyolitc and quartz state polymorph transistions. From that, you have 8 days. I don't call that weathering.

Then you go over to marine rocks. You get opal-A from forams. You slows bury this in muck and heat it to form opal-CT, linked to agate formation. You have up to 70 million years for the very coldest conditions of formation. That is it. So the relation is time-temperature dependent. But, the agates in those sedimentary rocks take a microscope to see. You have to understand that a 100 micron agate is not the same as a volcanic agates. For example, no dilation tubes of exit/escape exist on this scale. In fact, don't exist for any agates under about 3/8th of an inch. So the silica banding is different in these sedimentary "agates" which are more like veinlets of banded silica on a mm scale.

Since these form in subcritical conditions, these "agates" never have waterlines. They never have the calcite and celadonite and other mineral inclusions. They are another class. The scale of your classification matters, and overgeneralizing causes confusion because as you overgeneralize, you combine different geologic systems and then get a confusion of superficially conflicting data.

Overall, in volcanic systems, agates are forming in under 30,000 years. This is why you cannot dig in your back yard to study the soil weathering profile and find bigger agates with depth. There are no agates in weathering profiles.
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Donald Kasper March 02, 2017 04:07AM
When the Russians formed banded silica structures in platinum vessels in supercritical fluids in a few months using quartz sand, I wrote and asked if they could use bentonite instead. They laughed at me. I mean, why use silica with 800 grams per square meter surface area when they are perfectly happy using sand with 1 gram per square meter. Because surface area relates to reaction rate. This is why massive quartz and sand cannot make agate. The reaction chemistry is colloidal and in a colloid world it is all about chemical charge, surface area, concentration,and temperature.
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Donald Kasper March 02, 2017 04:19AM
A variety is a gem term of a colorized mineral of the same overall chemical composition.

Agates are full of calcite, clay, opal, moganite and silica polymorphs.

In infrared, agates have opal-CT and opal-C and I model three new species of opal. No agate on this planet has opal-A. Why not? Opal-A is the only opal formed with hydrothermal solutions and from weathering. I have a very specific key to identify opals in infrared and can see mixed forms which occur all the time.

In infrared, agates have two populations of moganite. I model one is alpha-moganite, the other is beta-moganite. Raman reports they are both moganite and cannot tell alpha from beta. The alpha is only in the banding, and the other is immiscible in the agate, and found in specific structures such as microspheres.

In infrared, I have proposed that beta-silica minerals can be found, and have shared that data with Caltech, Pasadena Planetary Science group. If correct, agates have beta-quartz. No beta-cristobalite, but that is found in geode shells. But waterlines can have opal-BC (opal-beta-cristobalite). In fact, the only place geologically you can ever find an opal-C in volcanic systems, is in the waterline structures of geodes/amygdules. No where else. The opal is a supercritical transition opal form and lives right there.

Agates have celadonite and bentonite. Foggy white agates can be beta-moganite rich or can be calcite-agates.

So these are multi-mineralic and as such are rocks. They are very complex rocks of mixed composition of many quartz and clay polymorphs. The minerals occur in specific structures that you can find if you use spot reflectance infrared to aim specifically at them.

Claims agates are 99% quartz is a farce, and Moxon needs to stop promoting that. He is saying that if you bash you agate to bits and pick out the quartz, it is very pure quartz. But, this is not what an agate structure specimen is, and falsely dumbs down the conversation to promote this varietal garbage. Agates have as little as about 40% quartz, which makes them rocks. This means I am talking about the actual, whole specimen, not the banding only. The inclusions tell the rest of their story of formation as each included mineral has specific, known conditions of formation.



Edited 1 time(s). Last edit at 03/02/2017 04:21AM by Donald Kasper.
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Donald Kasper March 02, 2017 04:25AM
Very good. The southern Brazil/Paraguay region is full of big amygdules, I must be wrong and you proved it. How old are these lavas? Cretacecous. Okay, where was South America in Cretaceceous? Farther south in a desert belt. Okay, what is the brown staining? Humic acid leaching from being moved up to the equator where plant matter in groundwater stained them. Now you know.

Would you like for me to tell you where Germany was in Permian? Okay, if the shells of geodes are kaolinized and the agates are full of kaolinite, those are 300-400 million year old agates and that was weathering.
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Donald Kasper March 02, 2017 04:31AM
Go to any hydrothermal vent site on this planet spewing out silica and hot water, and show me an agate. You cannot. Your hot water and silica model just died. I collect for myself. When I get BS in the literature I can check it in the field as I live just south of one of the largest agate trends in the world. Beware of PhD's writing about agates in playa lakes while living in Pennsylvania in coal country. Really, they should study ferns and leave that agates alone.

The Milford, Utah vent site is world famous. I have been there. I have samples. Opal-CTA and Opal-CT. No agate. No agate or jasper on this planet has opal-A, the only opal formed by weathering and surface hydrothermal deposition. Infrared proves it. You just have to look.
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Jolyon & Katya Ralph March 02, 2017 07:43AM
Donald,

That's a lot of messages but you haven't addressed yet (or rectified) your provably false statement:

"Agates form in volcanic rocks or volcanic ash related to volcanism, only. So-called sedimentary agates are found under ash beds that was the silica source. "


note: in your MANY previous messages you seem to reference previous statements by people without quoting what you are actually replying to, hence it's impossible to follow the thread of your responses. Remember the 'REPLY' button under a message simply replies to the whole thread, not an individual message.

I do, however, support your proposal to redefine agate as a 'rock' rather than a variety of chalcedony! It's no more pure chalcedony than flint is.
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Jolyon & Katya Ralph March 02, 2017 07:45AM
There are also plenty of sedimentary agates with no celadonite association, Perhaps you'd prefer to call them banded cherts, but they exist and are generally classified as agate by most people. If your intention is to redefine the nomenclature of agate to exclude those of purely sedimentary formation then that's an entirely different battle.
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Jolyon & Katya Ralph March 02, 2017 03:27PM
Thinking about this further, although there are some reasons for categorizing agate (and chalcedony in general) as a rock rather than a variety of quartz, there are equally good reasons for keeping it as it is.

It is essentially a 'monomineralic' rock with impurities that can be significant, but the presence of such impurities doesn't specifically exclude it from being regarded as a variety of the mineral species.

For example: sand calcite, desert rose gypsum / baryte all have significant SiO2 included, but that doesn't necessarily mean they should not be classed as varieties of their parent species. Even old favourites such as 'rutilated quartz' are clearly not themselves a rock (they are a component of a larger rock, eg a pegmatite) but are not entirely monomineralic either.

So, we are left with "where does it fit best into mindat?" And at least for now that is as a variety of quartz.

Jolyon
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Alfred L. Ostrander March 02, 2017 05:15PM
Thank you Jolyon for reconsidering your statement about agate as a rock. This is a situation where petrology and mineralogy should act hand in glove for the betterment of our understanding of quartz and its polytypes.

To the point of this posting. Mr Kasper, you have written a number of books about agates, geodes, etc. Many of your positions are well known and not agreed with. You do seem to have a habit of dropping some interesting lines.

Here is what Mr Kasper thinks on a few things.

Hardness and specific gravity have not been used in science since 1910 to identify minerals or rocks reported in the scientific literature.
There is a reason, and the reason is that those methods are grossly inaccurate.
Feb 21, 2013, Yahoo Group
Here is what Mr Kasper thinks of Mindat and Dana.

Oh well. I guess Mindat and the IMA is stuck in 1830, the age of Dana. It was such a simple world then. I see increasingly that our geology
books are more like history books of old science than real science. Many authors are lazy pigs, and quote that ancient data because it
keeps things simple, but the goal of science is to describe what we see and understand and understand with our best available data and
instrumentation, not the data of 1830. Dana is dead and the science has moved on.
Feb 20, 2013, Yahoo Group

Mr Kasper finished his post of Feb 21, 2013:

Lastly, I have never been ashamed of being smart.

I will also point out that on another posting, and I paraphrase, Mr Kasper made it clear to a researcher that when he had finished publishing as many books as he had, then they could compare notes.

Jolyon, if you wish, you can remove this. I am wondering if this thread should be closed.
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Gregg Little March 02, 2017 09:06PM
Jolyon;

I find Mr. Kasper ramblings very difficult to follow as he jumps around in the geological realm going from snippets of scientific investigations to depositional environments to post-depositional processes with very little in research back-up or references or collaborating visuals. As Alfred points out he is absurd in attempting to discredit foundational work and basic analytical procedures successfully used by hundreds to thousands of trained field geologists in the resource exploration industries to this day.

Mr. Kasper also confuses geological terminology through out the thread; as you pointed out with mono-mineralic and poly-mineralic rocks. In the ground water segment he appears to readily dismiss all but magmatic waters. In near surface environments the complex chemistry of "mixed" magmatic or juvenile, hydrothermal, connate and meteoric waters is well know for influencing mineral deposition. To further cloud the conversation he keeps introducing weathering which in the context of agate formation here is irrelevant (post depositional). Sweepingly introducing continental drift and geological ages to conveniently support his climate assertions is another ruse.

And finally one more absurd derision. Mr. Kasper appears to think that where you live determines what you can be an expert on "Beware of PhD's writing about agates in playa lakes while living in Pennsylvania in coal country.". Until reasoned discussion returns, Mr. Kasper's writings are becoming untenable.

I am with Alfred as he has offered, "Jolyon, if you wish, you can remove this. I am wondering if this thread should be closed.".
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Daniel Bennett March 02, 2017 09:52PM
they say you truly understand something when you can explain it to a kindergartener. i realize no one agrees about this then they have to try and outdo each other with jargon. for fun lets try to keep it simple here?
i propose not closing it yet because i have more simple questions and pictures to talk about but i need a little time to articulate them. also other people may appreciate it. and does it really solve a problem to close it?
thanks for your tolerance.
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Gregg Little March 02, 2017 11:26PM
Sorry Daniel, carry on asking and articulating as that's what we are up for, as opposed to violent eruptive discourse, ranting in a geological vein or depositing absolutes. We take a lot for granite but the discussion should have a gneiss demeanor.
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Alfred L. Ostrander March 03, 2017 01:21PM
Daniel,

My comments were tough. I will ask you this: Do you want good science or not? And are you in an informed enough position to understand some of the complexities of the arguments against some of the broad and sweeping statements that have been made?

As to understanding something so thoroughly that you can easily explain it to kindergartners, I don't think this applies to the the very real complexities of agate formation. Initially you proposed what you considered a logical and workable hypothesis. As logical as it sounded it was not based on good geologic evidence. At least you asked and I respect that. Parts of this thread have involved sweeping statements that even Jolyon questioned. Some really sharp geologists have raised serious questions in response to these sweeping claims. The difficult part of this lies in the fact that Mr Kasper has done a lot of work and has a lot of data. What is in question is his interpretation, his presentation and apparent disdain for the foundations of mineralogy and geology in general. Shall this continue without question and what would you really learn if such tenuous premises are left standing? What would anyone else learn if they come across this thread and accepted unproven conjectures as sound science?

I seriously doubt the positions taken here are simply efforts to outdo each other with jargon. However, it might be difficult to follow what is going on if you do not have the background in things geologic. How then do you sort out the false from the true? I presented comments from researching Mr Donald Kasper relating to his own statements to help clarify that he can get off point and venture off into unsubstantiated rhetoric. That is why I asked if this should come to an end. Please do not think for a moment that I take any pleasure or fun calling out bad science. I mostly find it onerous. Quite frankly, I far prefer being out in the field wearing out another good pair of boots.

Please continue to ask more questions. If not directly related to agate formation might I suggest new threads to clear the deck, so to speak. Just a thought to consider. J. D. Dana is physically dead. Science has progressed. J. D. Dana and his work still stand as an example of fine work. I ask no pardon for being a bit prickly in light of claims regarding his "death".
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Daniel Bennett March 03, 2017 06:11PM
please know I mean no offense to anyone with this. here is a bad picture of many air pockets in basalt with every ten or so being filled in as an agate. I would think a quartz ball makes a similar pocket as a gas bubble in basalt. if the agate formed from water/silica mixture penetrating the basalt then I would expect all air pockets should be filled in with agate. instead this picture implies that the quartz balls were swirling around in lava along side with gas bubbles.
it could be that they were all filled in and most have fallen out already but I don't think so. some pockets are %75 intact and empty.

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Kyle Beucke March 03, 2017 07:03PM
Daniel, I don't agree with that assumption. I have seen porphyritic, volcanic rock that has been hydrothermally altered and where some open spaces have been filled with hydrothermal minerals (sulfides, quartz) and others were not (or less-so). Altered rocks with replacement minerals or minerals filling open spaces are not uniformly filled-in with these minerals. The fluids travel through minute spaces (between crystals/grains?), not necessarily forming distinct veins, so it can be impossible to know where the fluids traveled.

Plus, chalcedony is a low-temperature silica phase, right? Not something I would expect to form in lava/magma.

Kyle
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Gregg Little March 03, 2017 09:16PM
Good question Daniel. There are at least a couple of overriding factors at play here and a little background helps clear this up. To start with it is generally acknowledged that chalcedony is formed or deposited in liquid or, gas to liquid phases (THE AMERICAN MINERALOGIST, VOL. 46, JANUARY_FEBRUARY, 1961., SYNTHESIS AND ORIGIN OF CHALCEDONY). This usually means the lava flow has solidified, freezing the gas bubbles in place. The lava can still be hot as fluid basalt has been recorded at 750 to 1000 C at Kilauea.

Generally chalcedony deposition occurs at relatively low temperature (180 to 300 C) and pressure (near surface) in what is usually termed a hydrothermal system which is the plumbing system that allows fluids to flow through the rock via fractures and voids like the gas bubbles (vesicles) in your picture.

That said, the liquid or gas bearing the silica in solution has to have access to the vesicles which is normally by fractures often form during cooling (shrinkage) of the lava flow. It is plausible that not all the vesicles would be intersected by fractures so when the hydrothermal system kicks in, some of the vesicles would be by-passed. Even if later the vesicle was fractured (further cooling, tectonic movement), the hydrothermal fluids could be depleted in dissolved silica and then other minerals, like calcite, could deposit instead or the space could just be devoid of minerals.

So there are multiple conditions to be met for chalcedony formation least of which is temperature, pressure, fluid chemistry, opportunity (fractures) and lava composition. Even the water that drives the hydrothermal system is complex with at least two possible sources; juvenile or magmatic (from the lava itself) and precipitation (meteoric). Meteoric water is usually neutral to mildly acidic so it would have to be changed by the rock it flows through to a mildly alkaline solution which is what chalcedony forms in. Very complex so I hope I wasn't too wordy or convoluted.
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Daniel Bennett March 03, 2017 10:32PM
thanks well that makes sense if it gets there through cracks rather than just flowing "through the rock". shouldn't there be seams on the agates from the cracks ? or agates nearby each other might be connected by excess chalcedony still filling the cracks. has that been observed in basalt? I haven't seen it but that would confirm what your saying for me. it would prove it...
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Wayne Corwin March 03, 2017 10:59PM
Daniel

It doesn't need cracks, the water can pass right thru the rock ;-)
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Donald Kasper March 04, 2017 12:03AM
You can advance infrared spectroscopy, and identify refractive index in the graphs of all minerals, in two places. Then from that, you can map all the silica minerals with those spectral effects. Then you will discover that you have about twice as many spectral band positions as you have silica minerals. I will name all of these after members of my family as new mineral species polymorphs of silica, else, these are the beta mineral polymorphs. Then once you find them, you go over to ceramics research and get their graphs of high temperature silica polymorphs in infrared. Everything matches. The volcanic rocks and the ceramic glasses have the same bands linked to refractive index. They are invariant. You get one, you know what you are seeing. This is called modern infrared. You have to advance the technology, then an artifact of that is you can advance the study of volcanic systems and the agates they contain.

So that disproves the consensus that beta-mineral forms all invert to alpha. In fact, discussing this with the senior professor of mineralogy at Caltech, I did ask if he had any proof all inversion is 100%, and nothing can be left behind as beta. He said no. Of course, we have coesite in the Alps, still identified with spectroscopy such as infrared, and I have such a specimen with a signature coesite band. Up 20 km, still coesite. So the notion high temperature and pressure polymorphs do not exist at the surface, is false.

Okay, you have a problem. There is beta-quartz in your agate dilation structures. 1 millimeter away it is 10% alpha-quartz in the banding. Explain that.

Tubes of escape from exothermic heat.

Tubes of entry to leach in silica sitting in dilation structures was just disproven. Beta-quartz forms at 575 C.

Some have beta-moganite, formed at 354 C.

The least squares regression graph correlation infrared spectral data in a linear fit has been generated, and has a correlation coefficient of 0.998 correlating refractive index to spectral data of all silicate minerals, and silica ones themselves as well. This statistical number ranges from 0 (random noise) to 1 (perfect fit to the line). This is a discovery of a law of physics. Fight it if you want, but you are going to lose. Armed with that, we have beta-silica minerals in agates in specific structures.

Two polymorphs of quartz in one specimen is the geologic definition of a rock. The varietal bunk has to go. We need to upgrade the conversation.



Edited 2 time(s). Last edit at 03/04/2017 12:09AM by Donald Kasper.
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Gregg Little March 04, 2017 12:37AM
True fluids can pass through seemingly solid rock. In the basalt flows on the Bay of Fundy (Nova Scotia) we often see seams of agate/chalcedony filling larger fractures. The agate may actually be in your location but because the fracture space is so small and the rock weathering, it might be overlooked in the crumbling material.

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Donald Kasper March 04, 2017 09:07AM
The purpose of science is to create conceptual models that explains the natural component under study for purposes of predicting natural systems behavior. If I develop a complex geologic model that explains everything seen, accounts for the literature of what others have seen, and explains it all, then I consider my model a superior model.

To achieve this, when you hit roadblocks in understanding it can be that the science itself has not advanced enough to provide an answer. Understanding agates is a byproduct of a superior understanding of volcanology, petrology, and instrument data such as the nature of the behavior of infrared spectroscopy.

So, when they say supercritical fluids start at 374 C and don't say what the upper temperature limit is, I know there must be an upper limit as supercritical fluids are not found on the sun. So I work down from there. Then I ran across chemical engineering and metallurgy research that defines yet another state of fluid matter they named ultra-supercritical water. It starts at 575 C, and they have coal boilers using this up to 610 C. Okay, what is the upper limit of supercritical water? 575 C. Okay, that is a quartz transition. Let us pen in that the crystal structure changes on fluid transitions, and this is why it does so. What is the next silica transition? 870 C. Okay, let us pen in an upper limit of ultra-supercritical water at exactly 870 C.

We just explained key silica polymorph transitions. Things are looking interesting.

Then we roam over to ceramics research and study the formation of optical quartz in a lab. They use high pressure autoclaves. What is the temperature they use to make optical quartz? 354 C. Right on the beta-moganite transition on the low end of the supercritical fluid transition. Why no higher? Because then you get contaminated quartz. What is contaminated quartz? Quartz with Brazil twinning. Okay, what is documented in all the literature is a key component of making banded agate? Brazil twinning. Okay, how does this jive with your weathering model? Not well.

Then if you heat the quartz, it changes to another twinning state, Dauphine twinning. Where does this live? Its temperature is well documented, and lives at exactly 575 C. Okay, what is the upper limit of making an agate? 575 C. Agate has no Dauphine twinning. This is the end of the line for agate. You have no physics to explain Brazil twinning at 10 C surface conditions, and ceramics chemists already know where it lives.

We just impugned on your weathering model, explaining a huge chunk of the physics we see and we are building a model of volcanology.

Let us not forget petrology. What is the meaning of metasomatism, medium grade metamorphism, and fluid-rock interaction? They are synonyms for supercritical fluid geochemistry. Look up the temperature ranges studied for this metamorphism. This is where it lives.

What is the meaning of high-grade metamorphism? Ultra-supercritical fluid geochemistry. Above 870 C, the water dissociates completely into H and OH. This is the end of high-grade metamorphism, and it is controlled by what water does.

So, where does rhyolite live? Ultra-supercritical fluid. This is why rhyolite lives here. The state of rhyolite from liquid to glass, the crystallography of the silica, the twinning that occurs, the metamorphism we see, is all the same story.



Edited 4 time(s). Last edit at 03/04/2017 09:35AM by Donald Kasper.
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Donald Kasper March 04, 2017 09:39AM
Any model of agate formation has to account for vein agate, amygdule, and geode formation.

Amygdules and geodes form in geologically closed systems on a scale of inches. Vein agates form in open systems, and can form in contact with the surface. As such, the fluids are not expected to be supercritical, and formed at and just under supercritical temperature. Except for a small handful of vein agates in the world, they will not have waterlines. When they do, they form perpendicular to the outer wall surface in vertical veins or nearly so. Algal structures can be found in some, along with surface mud, clay, and sand. Yet, the exterior shells are celadonized in all these systems.
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Donald Kasper March 04, 2017 09:50AM
Lava degasses upon coming to the surface from decompression, and the silica and feldspar separate. The silica comes from the melt. If you classify all silica minerals in infrared, then all the feldspars, you will see that lavas are primarily silica-feldspar exsolution systems. These are the two overwhelmingly dominant minerals seen with a method of spectroscopy such as infrared. Trace mineralization is not apparently controlling these lava systems.

After classifying all the feldspars in infrared, I studied the geodes. The lava shells represent silica capture of the magmatic host rock they formed in, so those shells tell us the lava and conditions the agates formed in. Armed with infrared spectrsocopy, you will discover no plagioclase feldspars in volcanic geodes. Only potassium feldspars.

in a weathering model, the host rock is irrelevant, and therefore this behavior is not explained by a weathering model of silica and water dumped about in opportunistic voids of any kind, but we already know that 1300 miles of the Sierra Nevada granites have zero agates, so water, silica, and voids cannot explain this omission.

In the geode lava shells, there is just one group of feldspars to be found in infrared--potassium feldspars. All except microcline, which is only found in granites. Why potassium feldspar? This group is the exsolution king, and when you talk about lava exsolution you are talking about potassium feldspars. For example, the solvus point for sanidine to exsolve is 575 C, at the liquid to glass state transition of rhyolite. This changes the viscosity of the lava and accounts in part for the changes from liquid to a fluid glass.

So, if you have a geologic world that only has potassium feldspar lava shells, known for silica-feldspar exsolution behavior, why do you have to worry where the silica came from? It is right there.



Edited 1 time(s). Last edit at 03/04/2017 09:51AM by Donald Kasper.
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Donald Kasper March 04, 2017 09:59AM
If you collect geodes from Fallen Tree, Oregon, the shells and voids are filled with mordenite and clinoptilolite. The agates however, do not contain them, in their silica. They are in their central voids, but again, not in the agate silica. The hydrothermal action came after the agate formation dropped dead. No silica came later to fill those central agate vugs and mordenite with more silica. Not one. How so in a weathering world of 10 C deposition? You tell me. I don't model agates formed from weathering.
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Donald Kasper March 04, 2017 10:11AM
Where hydrothermal fluids flow, you get bleaching of the host rock, which shows the channelways. This does not exist in lava flows with geodes. No alteration, no flow. Period.

When hydrothermal fluids do, in very exceptionally rare cases, enter into silica systems such as agate structures, we can see it, because we get supercritical fluid etching of embayments of the quartz, and the hydrothermal fluid with 1,000 times more carbonate than silica in solution, dumps a load of massive calcite as its calling card to let you know it was there. There are Scottish agates with embayments chopping perpendicular through the agate banding, and filled with calcite, for example. This observation has confused people back to the time of Heddle, who pondered over this problem.

Hydrothermal fluids dump calcite, not silica. Silica is almost insoluble in water. Calcite is highly soluble. This is why surface and groundwater circulation makes caliche, not agate. Caliche is not found in agates. The Baker Mine, NM has caliche pendants in the central vugs inside the agate and waterlines, but never in any of it. I did find, in 20 years study, one agate from the Santa Monica Mountains, CA with a floor of caliche. That is as close as I have gotten. However, the Baker pendant silica is crystallographic quartz crystal jumbles, every one apparently connected to a fissure and the area also has cones from venting silica into the voids. Vented silica makes cones, bigger becomes "chalcedony roses" which are really agates, and full coating in voids becomes Ocho agates from Paraguay. These are feather agates, where the banding formed, then deformed and tore into curled segments with very strong Brazil twinning sitting between banded agate and stockade quartz structures.

To have groundwater you must have humic acid. Hydrothermal water is heated groundwater. Therefore, universally, agates should be filled with a humic acid signature, which infrared is very sensitive to. I have bad news. Only a few rare sites, and only in the shells from weathering leaching into formed agate. Most agate water is in the shells, not the cores. Humic acid is in the shells, not the cores. The observations dispute a weathering model.

Primary magmatic calcite is never massive, it is scalenohedral, a form documented as the key state in mixed calcite-silica systems going back to observations geologist made in the early 1900's in New York.



Edited 1 time(s). Last edit at 03/04/2017 10:16AM by Donald Kasper.
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Knut Eldjarn March 04, 2017 10:24AM
Interesting discussion even if somewhat confusing. Banded chalcedony witha quarz/moganite composition seems to occur in different geological environments - and could probably be caused by different processes even if a vulcanic origin seems to be the vastly most common.
I just returned from a week in sunny Gran Canary and visited the type locality area for moganite. The silica-minerals quarz and moganite seems to be more sequencely separated in these ignimbrite localities. I wonder if this could be caused by the combination of under-cooling of parts of the silica containg fractions causing the crystallization of purer moganite, and if such under-cooling and low-temperature solidification could be caused by the physical forces of the pyroclastic flow creating the ignimbrite? In the case of these localities with many layers of lava flows below and above the ignimbrite, it is clear that the chalcedony/moganite/"agate" formation in the ignimbrite must have been related to this specific lava flow since I did not find a similar mineralization in the lava flows above or below the ignimbrite flow - even if their were many voids and cracks that could have been subject to such silica-formations if caused by post-vulcanic hydrothermal processes.
Any ideas or comments to these observations?

Knut



Edited 2 time(s). Last edit at 03/04/2017 03:45PM by Knut Eldjarn.
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Larry Maltby March 04, 2017 04:29PM
Knut,

I agree, much of the dialog above is confusing, some interesting insights mixed with strange conclusions. As you point out moganite likely plays a significant role in what we observe in agates. For me, the formation of agate that I observe in sections cut through basalt is the sum total of all of the processes that it has been exposed to over geologic time including secondary depositions and alterations. I became interested in moganite about three years ago and researched as much information as I could find. One professional paper stated that moganite was seven times more soluble than regular quartz. Another study regarding the solubility of quartz suggested that as a solvent exceeds a PH of about 9 the solubility of quartz increased significantly.

I do see much evidence of dissolved or leached bands in agates in the Copper City Flow in Houghton County, Michigan. Sometimes the bands remain open and sometimes the leached bands are replaced with hydrothermal copper. It would seem that a reasonable geological environment would be dissolution of moganite bands in agate by a hydrothermal base and then a change in the hydrothermal solution resulting in the deposition copper and the recrystallization of quartz.

You can see the amazing diversity of mineralization in the amygdaloidal basalts of the Copper City Flow at this location:

https://www.mindat.org/article.php/1801/+St.+Louis+Mine%2C+Houghton+Co.+Michigan

Daniel,

Here are some photos that may help you understand how fluids move through rocks and agates. The first one shows a micro fracture in basalt that passes through an agate and transports hydrothermal copper and quartz resulting in a “healed” fracture in the agate. Note that there are minute dots of copper throughout the basalt matrix indicating as Wayne said that fluids can pass through microscopic openings in the rock.
(Tom Rosemeyer specimen)

The second one shows a passage between two amygdales. Both have pink quartz rims but the lower has secondary pumpellyite and the upper has secondary laumontite. (Larry Maltby specimen)





Edited 2 time(s). Last edit at 03/04/2017 05:17PM by Larry Maltby.
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Gregg Little March 04, 2017 10:53PM
I concur with Larry Maltbywhen he says, "I agree, much of the dialog above is confusing, some interesting insights mixed with strange conclusions.". Again, no supporting visuals, tables, graphs, charts, scientific references, etc. Again Mr Kasper brings up the extraneously issue of weathering and once again I don't understand why a post depositional process is invoked then discounted when it is not involved in the agate forming process in the first place.

I offer quotes from scientific abstracts;

From "Contributions to Mineralogy and Petrology, March 1993, Volume 115, Issue 1, pp 66–74, A Proposed Mechanism for the Growth of Chalcedony":
The structural disparities that distinguish chalcedony from macrocrystalline quartz suggest that different crystallization mechanisms are operative during the growth of these two forms of silica. Although the paragenesis of chalcedony has provoked marked disagreement among researchers, a review of previous studies supports the idea that chalcedony can precipitate from slightly saturated aqueous solutions at relatively low temperatures (<100° C).

THE AMERICAN MINERALOGIST, VOL. 46, JANUARY_FEBRUARY, 1961 SYNTHESIS AND ORIGIN OF CHALCEDONY* J. F. Wurrn eNo J. F. ConwrN, Departments of Geology and Chemistry, Antioch College Yellow Springs, Ohio. Ansrn.qcr: Synthetic chalcedony has anomalous properties similar to those of natural chalcedony. These properties have been explained by submicroscopic holes and inferred disordered regions between fiber interfaces. The properties and origin of synthetic chalcedony are compatible with these concepts. The chalcedony was made by transformation of solid silica in the presence of hydrothermal solutions at moderate temperature and pressure. Chalcedony was not directly precipitated from solution, but formed only by transformation of silica glass or cristobalite. In general, no conversion took place in slightly acid solutions, while complete, rapid conversion occurred in slightly alkaline solutions. The transformation proceeds indirectly by way of cristobalite and keatite. Chalcedony is regarded as a secondary, metastable, transitional phase' The peculiar properties differentiating chalcedony from ordinary quartz may be a result of nucleation and growth in solid material (silica glass, opal, silica gel, or cristobalite).
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Ralph Bottrill March 05, 2017 02:54AM
Donald you raise some interesting questions and ideas, but they get lost in a hyperbolic diatribe against other researchers not here to defend themselves. You indicate there is a lot of supporting data, so you really need to collate this systematically and submit it to a scientific journal for peer review. Despite you feeling you are the only person in the world who understands the subject properly, most scientists are very interested in new concepts and interpretations and will welcome such research if it can be logically presented and substantiated. But you will have to carefully review the opposing views and refute them if you can. There seems to be an overwhelming support for relatively low temperature formation, but if you can support beta quartz formation in agates, supercritical fluids in sediments, no groundwater in deserts, etc, then the data needs presenting in a peer-reviewed journal, not in long rambling posts in Mindat. Though we welcome seeing some photos, references, etc.

It's instructive to compare agates with related materials. They are often difficult to distinguish from other chalcedony, except for the banding, and notably chalcedony often seem to replace opal. Agate and opal can form in cavities in petrified wood, and in cavities in silicified dolostones with no sign of volcanics. Chalcedony can infil and replace many permeable or reactive sediments, sometimes in hydrothermal systems, sometimes due to kaolinisation of feldspars releasing silica, or recrystallisation of volcanic glass, with no sign of high temperature alteration. 

But the banding in agate is the difference. It often seems to start as small botryoidal to stalactitic growths on the wall of the vesicle or cavity, (though often predated by clays), eg. https://www.mindat.org/photo-301303.html, which coalesce to form bands mostly parallel to cavity walls. Or horizontal, presumably due to a gas/water interface. But you can very commonly see a spot or two where the bands join at a feeder point, where the silica-bearing fluids entered the cavity, eg. https://www.mindat.org/photo-84206.html. In thin section you can see microfracturing in the host rock with commonly some fine chalcedonic filling. None of this proves a high or low temperature of course, but does indicate a slow, sustained, rhythmic (seasonal?) growth, with fracture control of permeability as Larry indicated. There is no evidence for exsolution or rapid dumping of masses of silica as far as I know. You might expect supercritical fluids to exsolve rather rapidly from volcanics and form coarse crystals, not hugely protracted microcrystalline growth.

You can get agate like textures in calcite in travertine, cave formations and in volcanic rocks. Those are usually considered very low temperature. Ditto malachite, siderite and many other carbonates, plus goethite, hematite etc. These seem to be due to slow growth in a damp, but not water-filled, environment, while we find in permanent pools in caves we often get, in contrast, well formed crystals.  So I guess that's an appropriate analogy for agates, damp cavities relatively rapidly getting coated in silica (maybe opal at first), and sometimes when later filled with water, the macrocrystalline quartz may grow more slowly?

But I'm no expert, so welcome other ideas, preferably with supporting data?

Whether agate is a rock or mineral is another important discussion. Its like travertine, chert, flint, jasper, limonite, marble, serpentine, manganese oxides, etc., these can all vary from monominerallic to complex mixtures. It does seem inconsistent to call something a mineral when it can contain sub equal amounts of two or more distinct minerals, while eg. quartzite is usually called a rock even when 99.9% pure quartz. The distinction on a specimen or field basis usually comes down to a definition of minerals being visually homogeneous materials forming small scale structures (eg crystals, geodes, veins, phenocrysts, porphyroblasts, etc) generally post dating the main host rock. (Yes I know phenocrysts predate the host rock!). If it forms large rock units then it's just a rock. But materials called a mineral in this field classification may actually be mixtures when analysed, so then should then really be labelled a rock. Though we make exceptions for things like sand calcite, or rutilated quartz, where the host crystals are prominent and dominate the mineralogy. We could probably make a similar exemption for agates, based on morphology, but this gets very subjective and maybe we could include it as both a rock and a mineral, along with chert, flint, etc? In reality it's no different to listing marble and limestones under calcite, sometimes that's all they are, just on a large scale. Someday we may have a problem when we find agates dominated by moganite!
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Donald Kasper March 05, 2017 11:32AM
This is an amateur forum and is not sufficient for complete geologic answers. I think I cover my case in 22 books on geoscience, some 4000 pages, which I cannot condense here. Publication on infrared involved a paper submitted to a journal whose review was poor. It was rejected based on the apparent slight of a research paper cited that was perhaps the editors colleague, while no rebuttal of a single statement in the paper was provided. Let me see. I proved the existence in natural systems of negative refractive index. While this prompted a call from a research fellow and world expert on infrared at NASA I sent this paper to personally review, the journal reviewer was uninterested. There is an issue of intellectual property rights granted to journals. In addition, it takes about 400 manhours to write a paper, and one flippant review to reject it, in my case, apparently by skip reading the top sentence of each paragraph, then asking questions answered in the next sentences. Your experiences may have been better. I propose some ideas here for others to consider, to expand their range of thinking.

Okay, here is one.

You have geodes with lava shells. There is a class of geodes with crystallographic structural cores. Some have biconoids, two radial cristobalite blooms attached on a common surface. Then you have cubic cores (box cores), each with a cristobalite bloom on each face. Then you have triconoids, each with a cristobalite bloom on a tetrahedral face. We know this as these cores can weather out of the shells for us to study. Central buttons and radial structures are typical of acicular cristobalite structures. Now, things get interesting.

If one were to propose that gas expansion makes these structures and then the voids fill with agate over a geologic age, say 75 million years since Tertiary, what physics can be proposed to explain a gas making crystal structures in lava? There is no such physics. Okay, this model is dead.

If one were to propose that there was a mineral that made these structures, which cannot be silica by a weathering model, what is it, and where did it go? No residue of foreign mineralization can be found in any void fill lining, and I have the instrumentation to aim and look. So prior mineralization and dissolution and void fill since Tertiary does not work. Okay, this model is dead.

So, what are we left with? These structures are consistent as morphologies in the same class as beta-cristobalite and beta-quartz. The cores are now agate. The silica went nowhere. What does this mean? That class of agates are low temperature inversion polymorphs of beta-cristobalite and beta-quartz. All that happened was the structure cooled down, and its crystal structure reorganized. Nothing went anywhere. Now all the data fits.

There was no weathering. There was no void stage. However, the shells of Oregon geodes commonly have beta-cristobalite and beta-quartz remnants.



Edited 1 time(s). Last edit at 03/05/2017 11:34AM by Donald Kasper.
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Donald Kasper March 05, 2017 11:46AM
How do you get lower temperature agates? You mix supercritical CO2 and supercritical water. The inversion point is based on the ratio of the mix of each supercritical fluid. Calcite cannot be defined without stating the partial pressure of CO2 when discussing its formation geochemistry. CO2 supercritical point is 32 C. CO2 in water lowers the supercritical point.

We have dozens of variables going on, and to conceptualize can only describe two at a time. So no item discussion of two things at a point in composition and conditions covers the whole story.

There is a class of agates formed subcritical. They form slightly differently. None form from weathering. Crystal quartz does not form at 10 C. Brazil twinning does not occur at 10 C. There is no sticky physics to attach silica in water to any wall surface.

Agates cannot occur with boiling. They cannot form in saline systems. Then occur around pH 8.5, which is not groundwater pH. They cannot form in playa lakes. They have no evaporites or salts. If your silica system has salt, you get chert, and not agate.

When you have a basalt flow over a tuff flow and you want to hunt for jasper or agate, where do you look? Anywhere in flow because weathering made it? You look at the 2 inch contact of the basalt to the tuff and nowhere else. If not there, it is not in that flow. That is where the silica went supercritical. In fact, there is a class of jaspers/agates that have sediment capture floors and quartz tops. I have collected those for myself here in the Mojave Desert. All tuffs do not have the same composition, and as such agates and jaspers are not in all tuffs and all tuffs were not exposed to the same conditions, but all have been exposed to weathering since at least Tertiary.
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Donald Kasper March 05, 2017 12:09PM
Kaolinite only forms in acid systems, and from weathering. It can leach into the fiber layers of agates in wet climatic regions. Kaolinization is a key indicator of very old agates, usually Permian. More recent volcanics does not have kaolin. The clay in agates is from the smectite group only. Sepiolite is linked in many sites to purple chalcedonies. You cannot dig for agates in kaolinite tuffs to find agates.

You found agate, you look in the reported statigraphy for the bentonite. Let me see, someone found agate in shale. The Pierre Shale of South Dakota probably has agates as it also has 7 bentonite beds.

Any geochemical reaction that releases silica at a temperature too low to make silicates, is a candidate to make agate if water is also released in the process. For example, tuff zeolitization.

Agate vs chalcedony. Chalcedony is chert. These are sulfide-rich systems where agates cannot typically, and usually marginally form. Chalcedony is granular quartz. I don't recall seeing a chalcedony with opal.

Opal precursor and alteration. 75 million year old opal is still here. I think that is a good enough test for the spontaneous inversion theory. Let me see, Moxon proposed that moganite inverts to quartz, and put a limit on this. Few tens of millions of years as I recall off hand. Sorry Moxon, I have moganite in Kentucky agates of Mississippian age, 135 million years, in an agate. Was that enough time to invert? Apparently not. There is zero alteration and difference from modern moganite sites.

Agate in travertine. Yes. On a millimeter scale. As the CO2 load changes, calcite or silica comes out. There is that issue where that was on the surface, and probably was not. In cave aragonite? No, that is preposterous.

Goethite captured in agate of volcanic origin? No. In the outer aureoles of volcanogenic massive sulfide desposits mediated by cyanobacterial mats? Yes. These are supercritical water vents. Think Missouri agates around galena deposits, a long way from the galena. Outer aureoles are alkaline, but still hot to 95 C. Forms with barite. Sulfate is key, neutralizing the acid. These are not weathering systems. They are bacteria mediated systems.
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Donald Kasper March 05, 2017 12:10PM
Correct. This is not a research forum. Sharing some ideas with those interested. Those not can move along. Hundreds of manhours to show scientific proofs here is just goofy and presumptive.
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Donald Kasper March 05, 2017 12:34PM
If you label agate, quartz, the consequence is that the conversation becomes stupid. All the study of the inclusions that tell the rest of the story of agate formation drops out. To say a structure formed with celadonite, calcite, moganite, and opal is a chalcedony as quartz, then occurs. Then you get confused what the difference could be. Then someone comes along and calls agate 99% quartz when this almost never occurs and so again, all the inclusions drop out. Calcite chemistry is key to agate formation, that drops out.

What has occurred is that Moxon and colleagues, who I have corresponded with say, inclusions are not agate. Okay, study the quartz only in agate and see if it is pure. So what. This is irrelevant research. It also confuses one part of the agate with the whole specimen, which amateurs cannot discern in Moxon's/Frondel's definition. If you have an agate in your hand and it has inclusions where it is 40% quartz, why pretend it is a 99% pure variety of quartz, when this is false?

The definition gets at what you have collected as a specimen and what you want to study. The specimens we collected are almost never 99% quartz. This is a lie. It is a disgusting fraud and should stop. It is equivalent to saying houses are quartz because they have windows with quartz. The reason why the story of agates is not finished is due to this problem that the inclusions are irrelevant when they are totally relevant to the discussion.

To call an agate a chalcedony when chalcedonies lack most of the inclusion in agates creates a false equivalency. There is no opal-C in chalcedony (massive, granular quartz). No opal-CT. No scalenohedral calcite. No moganite microspheres or microdiscs on silica banding planes. No beta-moganite from supercritical fluid migration (ellipsoids). No beta-quartz. No celadonite, bentonite, nontronite, glauconite. No plumes, no tubes, no opal dendrites. Meanwhile the chalcedonies are rich in radiolaria, which is rare in marine influenced agates and nonexistent in volcanic agates. So what occurs is that when you overgeneralize terminology to overlap dozens of geologic systems, you cannot ferret out what is going on as the literature and discussion becomes confused. Because chalcedony has silica, then this approach means that cristobalite, another form of silica, is just a bunch of silica, and as such should be called a variety of chalcedony. Can you see the problem with that? Chalcedony is a rock of silica and sulfides. Agates are rocks of silica and a suite of opals, moganites, cristobalites, clays, carbonates. There is no merit to combine the two.



Edited 2 time(s). Last edit at 03/05/2017 12:38PM by Donald Kasper.
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Luís Martins March 05, 2017 04:21PM
This is starting to sound like a Trump speech...



Edited 1 time(s). Last edit at 03/05/2017 04:49PM by Luís Martins.
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Peter Nancarrow March 05, 2017 04:23PM
I find agates an interesting subject, and I started to read this thread trying (with increasing difficulty) to understand just what concept was being discussed here, whether there was a substantive new theory of agate formation to be assesed and taken seriously, and if so could I learn anything from it.

However, having stumbled through a maze of geologically erroneous statments, circular arguments and irrelevant ramblings, the final straw which confimed beyond reasonable doubt that I was wasting my time reading this thread were statements such as:

Two polymorphs of quartz in one specimen is the geologic definition of a rock.


I see no point in taking a thesis which bases its arguments on defintions blatantly and absurd erroneous as that at all seriously or engaging in debate with it!

Pete N.



Edited 3 time(s). Last edit at 03/05/2017 04:41PM by Peter Nancarrow.
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